Method for processing received RLC PDUs for D2D communication system and device therefor

ABSTRACT

The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for processing received RLC PDUs for D2D communication system, the method comprising: receiving a first RLC (Radio Link Control) PDU (Protocol Data Unit) for a RLC entity from a peer UE; setting a plurality of state variables for the RLC entity to initial values, respectively, wherein the plurality of state variables for the RLC entity comprises VR(UH) and VR(UR), and the an initial value of VR(UH) is a RLC sequence number (SN) of the first RLC PDU, and an initial value of VR(UR) is ‘the RLC SN of the first RLC PDU-offset’; and processing the first RLC PDU using the initial values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2015/003517, filed on Apr. 8, 2015,which claims the benefit of U.S. Provisional Application No. 61/989,495,filed on May 6, 2014, the contents of which are all hereby incorporatedby reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more particularly, to a method for processing received RLC PDUs (RadioLink Control Protocol Data Units) for D2D (Device to Device)communication system and a device therefor.

BACKGROUND ART

As an example of a mobile communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project Long TermEvolution (hereinafter, referred to as LTE) communication system isdescribed in brief.

FIG. 1 is a view schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system. An Evolved UniversalMobile Telecommunications. System (E-UMTS) is an advanced version of aconventional Universal Mobile Telecommunications System (UMTS) and basicstandardization thereof is currently underway in the 3GPP. E-UMTS may begenerally referred to as a Long Term Evolution (LTE) system. For detailsof the technical specifications of the UMTS and E-UMTS, reference can bemade to Release 7 and Release 8 of “3rd Generation Partnership Project;Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs(eNBs), and an Access Gateway (AG) which is located at an end of thenetwork (E-UTRAN) and connected to an external network. The eNBs maysimultaneously transmit multiple data streams for a broadcast service, amulticast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in oneof bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides adownlink (DL) or uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be set to provide differentbandwidths. The eNB controls data transmission or reception to and froma plurality of UEs. The eNB transmits DL scheduling information of DLdata to a corresponding UE so as to inform the UE of a time/frequencydomain in which the DL data is supposed to be transmitted, coding, adata size, and hybrid automatic repeat and request (HARM)-relatedinformation. In addition, the eNB transmits UL scheduling information ofUL data to a corresponding UE so as to inform the UE of a time/frequencydomain which may be used by the UE, coding, a data size, andHARQ-related information. An interface for transmitting user traffic orcontrol traffic may be used between eNBs. A core network (CN) mayinclude the AG and a network node or the like for user registration ofUEs. The AG manages the mobility of a UE on a tracking area (TA) basis.One TA includes a plurality of cells.

Device to device (D2D) communication refers to the distributedcommunication technology that directly transfers traffic betweenadjacent nodes without using infrastructure such as a base station. In aD2D communication environment, each node such as a portable terminaldiscovers user equipment physically adjacent thereto and transmitstraffic after setting communication session. In this way, since D2Dcommunication may solve traffic overload by distributing trafficconcentrated into the base station, the D2D communication may havereceived attention as the element technology of the next generationmobile communication technology after 4G. For this reason, the standardinstitute such as 3GPP or IEEE has proceeded to establish the D2Dcommunication standard on the basis of LTE-A or Wi-Fi, and Qualcomm hasdeveloped their own D2D communication technology.

It is expected that the D2D communication contributes to increasethroughput of a mobile communication system and create new communicationservices. Also, the D2D communication may support proximity based socialnetwork services or network game services. The problem of link of a userequipment located at a shade zone may be solved by using a D2D link as arelay. In this way, it is expected that the D2D technology will providenew services in various fields.

The D2D communication technologies such as infrared communication,ZigBee, radio frequency identification (RFID) and near fieldcommunications (NFC) based on the RFID have been already used. However,since these technologies support communication only of a specific objectwithin a limited distance (about 1 m), it is difficult for thetechnologies to be regarded as the D2D communication technologiesstrictly.

Although the D2D communication has been described as above, details of amethod for transmitting data from a plurality of D2D user equipmentswith the same resource have not been suggested.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method and device for processing received RLC PDUs for D2Dcommunication system by setting a plurality of state variables for theRLC entity to new state variables. The technical problems solved by thepresent invention are not limited to the above technical problems andthose skilled in the art may understand other technical problems fromthe following description.

Technical Solution

The object of the present invention can be achieved by providing amethod for a User Equipment (UE) operating in a wireless communicationsystem, the method comprising: receiving a first RLC (Radio LinkControl) PDU (Protocol Data Unit) for a RLC entity from a peer UE;setting a plurality of state variables for the RLC entity to initialvalues, respectively, wherein the plurality of state variables for theRLC entity comprises VR(UH) and VR(UR), and the an initial value ofVR(UH) is a RLC sequence number (SN) of the first RLC PDU, and aninitial value of VR(UR) is ‘the RLC SN of the first RLC PDU-offset’; andprocessing the first RLC PDU using the initial values.

In another aspect of the present invention, provided herein is a UE(User Equipment) for operating in a wireless communication system, theUE comprising: an RF (Radio Frequency) module; and a processorconfigured to control the RF module, wherein the processor is configuredto receive a first RLC (Radio Link Control) PDU (Protocol Data Unit) fora RLC entity from a peer UE, to set a plurality of state variables forthe RLC entity to initial values, respectively, wherein the plurality ofstate variables for the RLC entity comprises VR(UH) and VR(UR), and thean initial value of VR(UH) is a RLC sequence number (SN) of the firstRLC PDU, and an initial value of VR(UR) is ‘the RLC SN of the first RLCPDU-offset’, and to process the first RLC PDU using the initial values.

Preferably, the method further comprise: establishing the RLC entity toprocess the first RLC PDU when the first RLC PDU is received.

Preferably, the RLC entity is a UM (Unacknowledged Mode)-RLC entity andthe RLC PDU is an RLC UMD (Un Unacknowledged Mode Data) PDU.

Preferably, wherein the VR(UR) is a received state variable for a UM RLCentity and holds a value of RLC SN of a earliest UMD PDU that is stillconsidered for reordering, and the VR(UH) is highest received statevariable for the UM RLC entity and holds a value of RLC SN following SNof a UMD PDU with the highest SN among received UMD PDUs.

Preferably, the first RLC PDU is an RLC PDU received firstly before anyother RLC PDUs are received by the RLC entity from the peer UE.

Preferably, the method further comprise: setting the VR(UH) from theinitial value to ‘the RLC SN of the first RLC PDU+1’, when the first RLCPDU is stored in a reordering buffer.

Preferably, the method further comprise: starting a timer when the firstRLC PDU is stored in a reordering buffer; and setting a UR(UX) to ‘theRLC SN of the first RLC PDU+1’, wherein the UR(UX) holds a value of SNfollowing SN of a RLC PDU which triggered the timer.

Preferably, the method further comprise: delivering the first RLC PDU toa PDCP (Packet Data Convergence Protocol) entity when the timer expires.

Preferably, the method further comprise: receiving a value the offsetfrom the peer UE or a eNode-B by SIB (System Information Blocks), RRC(Radio Resource Control), PDCP (Packet Data Convergence Protocol), MAC(Medium Access Control) or PHY (PHYsical) signaling

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to the present invention, received RLC PDUs can be efficientlyprocess in D2D communication system by setting a plurality of statevariables for the RLC entity to new state variables. It will beappreciated by persons skilled in the art that that the effects achievedby the present invention are not limited to what has been particularlydescribed hereinabove and other advantages of the present invention willbe more clearly understood from the following detailed description takenin conjunction with the accompanying drawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram showing a network structure of an Evolved UniversalMobile Telecommunications System (E-UMTS) as an example of a wirelesscommunication system;

FIG. 2A is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS), and FIG. 2B is ablock diagram depicting architecture of a typical E-UTRAN and a typicalEPC;

FIG. 3 is a diagram showing a control plane and a user plane of a radiointerface protocol between a UE and an E-UTRAN based on a 3rd generationpartnership project (3GPP) radio access network standard;

FIG. 4 is a diagram of an example physical channel structure used in anE-UMTS system;

FIG. 5 is a block diagram of a communication apparatus according to anembodiment of the present invention;

FIG. 6 is an example of default data path for a normal communication;

FIGS. 7˜8 are examples of data path scenarios for a proximitycommunication;

FIG. 9 is a conceptual diagram illustrating for a non-roaming referencearchitecture;

FIG. 10 is a conceptual diagram illustrating for a Layer 2 Structure forSidelink;

FIG. 11a is a conceptual diagram illustrating for User-Plane protocolstack for ProSe Direct Communication, and FIG. 11b is Control-Planeprotocol stack for ProSe Direct Communication;

FIG. 12 is a conceptual diagram illustrating for a PC5 interface forProSe Direct Discovery;

FIG. 13 is a conceptual diagram illustrating for model of twounacknowledged mode peer entities;

FIGS. 14a to 14f are conceptual diagrams illustrating for UMD PDU;

FIG. 15 is a conceptual diagram illustrating for UM RLC reorderingwindow operation; and

FIGS. 16 and 17 are conceptual diagrams for processing received RLC PDUfor D2D communication according to embodiments of the present invention.

BEST MODE

Universal mobile telecommunications system (UMTS) is a 3rd Generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). The long-term evolution (LTE) of UMTS is under discussion by the3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3G LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

Hereinafter, structures, operations, and other features of the presentinvention will be readily understood from the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Embodiments described later are examples in which technicalfeatures of the present invention are applied to a 3GPP system.

Although the embodiments of the present invention are described using along term evolution (LTE) system and a LTE-advanced (LTE-A) system inthe present specification, they are purely exemplary. Therefore, theembodiments of the present invention are applicable to any othercommunication system corresponding to the above definition. In addition,although the embodiments of the present invention are described based ona frequency division duplex (FDD) scheme in the present specification,the embodiments of the present invention may be easily modified andapplied to a half-duplex FDD (H-FDD) scheme or a time division duplex(TDD) scheme.

FIG. 2A is a block diagram illustrating network structure of an evolveduniversal mobile telecommunication system (E-UMTS). The E-UMTS may bealso referred to as an LTE system. The communication network is widelydeployed to provide a variety of communication services such as voice(VoIP) through IMS and packet data.

As illustrated in FIG. 2A, the E-UMTS network includes an evolved UMTSterrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC)and one or more user equipment. The E-UTRAN may include one or moreevolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 maybe located in one cell. One or more E-UTRAN mobility management entity(MME)/system architecture evolution (SAE) gateways 30 may be positionedat the end of the network and connected to an external network.

As used herein, “downlink” refers to communication from eNodeB 20 to UE10, and “uplink” refers to communication from the UE to an eNodeB. UE 10refers to communication equipment carried by a user and may be alsoreferred to as a mobile station (MS), a user terminal (UT), a subscriberstation (SS) or a wireless device.

FIG. 2B is a block diagram depicting architecture of a typical E-UTRANand a typical EPC.

As illustrated in FIG. 2B, an eNodeB 20 provides end points of a userplane and a control plane to the UE 10. MME/SAE gateway 30 provides anend point of a session and mobility management function for UE 10. TheeNodeB and MME/SAE gateway may be connected via an S1 interface.

The eNodeB 20 is generally a fixed station that communicates with a UE10, and may also be referred to as a base station (BS) or an accesspoint. One eNodeB 20 may be deployed per cell. An interface fortransmitting user traffic or control traffic may be used between eNodeBs20.

The MME provides various functions including NAS signaling to eNodeBs20, NAS signaling security, AS Security control, Inter CN node signalingfor mobility between 3GPP access networks, Idle mode UE Reachability(including control and execution of paging retransmission), TrackingArea list management (for UE in idle and active mode), PDN GW andServing GW selection, MME selection for handovers with MME change, SGSNselection for handovers to 2G or 3G 3GPP access networks, Roaming,Authentication, Bearer management functions including dedicated bearerestablishment, Support for PWS (which includes ETWS and CMAS) messagetransmission. The SAE gateway host provides assorted functions includingPer-user based packet filtering (by e.g. deep packet inspection), LawfulInterception, UE IP address allocation, Transport level packet markingin the downlink, UL and DL service level charging, gating and rateenforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAEgateway 30 will be referred to herein simply as a “gateway,” but it isunderstood that this entity includes both an MME and an SAE gateway.

A plurality of nodes may be connected between eNodeB 20 and gateway 30via the S1 interface. The eNodeBs 20 may be connected to each other viaan X2 interface and neighboring eNodeBs may have a meshed networkstructure that has the X2 interface.

As illustrated, eNodeB 20 may perform functions of selection for gateway30, routing toward the gateway during a Radio Resource Control (RRC)activation, scheduling and transmitting of paging messages, schedulingand transmitting of Broadcast Channel (BCCH) information, dynamicallocation of resources to UEs 10 in both uplink and downlink,configuration and provisioning of eNodeB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE-IDLE state management,ciphering of the user plane, System Architecture Evolution (SAE) bearercontrol, and ciphering and integrity protection of Non-Access Stratum(NAS) signaling.

The EPC includes a mobility management entity (MME), a serving-gateway(S-GW), and a packet data network-gateway (PDN-GW). The MME hasinformation about connections and capabilities of UEs, mainly for use inmanaging the mobility of the UEs. The S-GW is a gateway having theE-UTRAN as an end point, and the PDN-GW is a gateway having a packetdata network (PDN) as an end point.

FIG. 3 is a diagram showing a control plane and a user plane of a radiointerface protocol between a UE and an E-UTRAN based on a 3GPP radioaccess network standard. The control plane refers to a path used fortransmitting control messages used for managing a call between the UEand the E-UTRAN. The user plane refers to a path used for transmittingdata generated in an application layer, e.g., voice data or Internetpacket data.

A physical (PHY) layer of a first layer provides an information transferservice to a higher layer using a physical channel. The PHY layer isconnected to a medium access control (MAC) layer located on the higherlayer via a transport channel. Data is transported between the MAC layerand the PHY layer via the transport channel. Data is transported betweena physical layer of a transmitting side and a physical layer of areceiving side via physical channels. The physical channels use time andfrequency as radio resources. In detail, the physical channel ismodulated using an orthogonal frequency division multiple access (OFDMA)scheme in downlink and is modulated using a single carrier frequencydivision multiple access (SC-FDMA) scheme in uplink.

The MAC layer of a second layer provides a service to a radio linkcontrol (RLC) layer of a higher layer via a logical channel. The RLClayer of the second layer supports reliable data transmission. Afunction of the RLC layer may be implemented by a functional block ofthe MAC layer. A packet data convergence protocol (PDCP) layer of thesecond layer performs a header compression function to reduceunnecessary control information for efficient transmission of anInternet protocol (IP) packet such as an IP version 4 (IPv4) packet oran IP version 6 (IPv6) packet in a radio interface having a relativelysmall bandwidth.

A radio resource control (RRC) layer located at the bottom of a thirdlayer is defined only in the control plane. The RRC layer controlslogical channels, transport channels, and physical channels in relationto configuration, re-configuration, and release of radio bearers (RBs).An RB refers to a service that the second layer provides for datatransmission between the UE and the E-UTRAN. To this end, the RRC layerof the UE and the RRC layer of the E-UTRAN exchange RRC messages witheach other.

One cell of the eNB is set to operate in one of bandwidths such as 1.25,2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplinktransmission service to a plurality of UEs in the bandwidth. Differentcells may be set to provide different bandwidths.

Downlink transport channels for transmission of data from the E-UTRAN tothe UE include a broadcast channel (BCH) for transmission of systeminformation, a paging channel (PCH) for transmission of paging messages,and a downlink shared channel (SCH) for transmission of user traffic orcontrol messages. Traffic or control messages of a downlink multicast orbroadcast service may be transmitted through the downlink SCH and mayalso be transmitted through a separate downlink multicast channel (MCH).

Uplink transport channels for transmission of data from the UE to theE-UTRAN include a random access channel (RACH) for transmission ofinitial control messages and an uplink SCH for transmission of usertraffic or control messages. Logical channels that are defined above thetransport channels and mapped to the transport channels include abroadcast control channel (BCCH), a paging control channel (PCCH), acommon control channel (CCCH), a multicast control channel (MCCH), and amulticast traffic channel (MTCH).

FIG. 4 is a view showing an example of a physical channel structure usedin an E-UMTS system. A physical channel includes several subframes on atime axis and several subcarriers on a frequency axis. Here, onesubframe includes a plurality of symbols on the time axis. One subframeincludes a plurality of resource blocks and one resource block includesa plurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use certain subcarriers of certain symbols (e.g., a firstsymbol) of a subframe for a physical downlink control channel (PDCCH),that is, an L1/L2 control channel. In FIG. 4, an L1/L2 controlinformation transmission area (PDCCH) and a data area (PDSCH) are shown.In one embodiment, a radio frame of 10 ms is used and one radio frameincludes 10 subframes. In addition, one subframe includes twoconsecutive slots. The length of one slot may be 0.5 ms. In addition,one subframe includes a plurality of OFDM symbols and a portion (e.g., afirst symbol) of the plurality of OFDM symbols may be used fortransmitting the L1/L2 control information. A transmission time interval(TTI) which is a unit time for transmitting data is 1 ms.

A base station and a UE mostly transmit/receive data via a PDSCH, whichis a physical channel, using a DL-SCH which is a transmission channel,except a certain control signal or certain service data. Informationindicating to which UE (one or a plurality of UEs) PDSCH data istransmitted and how the UE receive and decode PDSCH data is transmittedin a state of being included in the PDCCH.

For example, in one embodiment, a certain PDCCH is CRC-masked with aradio network temporary identity (RNTI) “A” and information about datais transmitted using a radio resource “B” (e.g., a frequency location)and transmission format information “C” (e.g., a transmission blocksize, modulation, coding information or the like) via a certainsubframe. Then, one or more UEs located in a cell monitor the PDCCHusing its RNTI information. And, a specific UE with RNTI “A” reads thePDCCH and then receive the PDSCH indicated by B and C in the PDCCHinformation.

FIG. 5 is a block diagram of a communication apparatus according to anembodiment of the present invention.

The apparatus shown in FIG. 5 can be a user equipment (UE) and/or eNBadapted to perform the above mechanism, but it can be any apparatus forperforming the same operation.

As shown in FIG. 5, the apparatus may comprises a DSP/microprocessor(110) and RF module (transceiver; 135). The DSP/microprocessor (110) iselectrically connected with the transceiver (135) and controls it. Theapparatus may further include power management module (105), battery(155), display (115), keypad (120), SIM card (125), memory device (130),speaker (145) and input device (150), based on its implementation anddesigner's choice.

Specifically, FIG. 5 may represent a UE comprising a receiver (135)configured to receive a request message from a network, and atransmitter (135) configured to transmit the transmission or receptiontiming information to the network. These receiver and the transmittercan constitute the transceiver (135). The UE further comprises aprocessor (110) connected to the transceiver (135: receiver andtransmitter).

Also, FIG. 5 may represent a network apparatus comprising a transmitter(135) configured to transmit a request message to a UE and a receiver(135) configured to receive the transmission or reception timinginformation from the UE. These transmitter and receiver may constitutethe transceiver (135). The network further comprises a processor (110)connected to the transmitter and the receiver. This processor (110) maybe configured to calculate latency based on the transmission orreception timing information.

Recently, Proximity-based Service (ProSe) has been discussed in 3GPP.The ProSe enables different UEs to be connected (directly) each other(after appropriate procedure(s), such as authentication), through eNBonly (but not further through Serving Gateway (SGW)/Packet Data NetworkGateway (PDN-GW, PGW)), or through SGW/PGW. Thus, using the ProSe,device to device direct communication can be provided, and it isexpected that every devices will be connected with ubiquitousconnectivity. Direct communication between devices in a near distancecan lessen the load of network. Recently, proximity-based social networkservices have come to public attention, and new kinds of proximity-basedapplications can be emerged and may create new business market andrevenue. For the first step, public safety and critical communicationare required in the market. Group communication is also one of keycomponents in the public safety system. Required functionalities are:Discovery based on proximity, Direct path communication, and Managementof group communications.

Use cases and scenarios are for example: i) Commercial/social use, ii)Network offloading, iii) Public Safety, iv) Integration of currentinfrastructure services, to assure the consistency of the userexperience including reachability and mobility aspects, and v) PublicSafety, in case of absence of EUTRAN coverage (subject to regionalregulation and operator policy, and limited to specific public-safetydesignated frequency bands and terminals)

FIG. 6 is an example of default data path for communication between twoUEs. With reference to FIG. 6, even when two UEs (e.g., UE1, UE2) inclose proximity communicate with each other, their data path (userplane) goes via the operator network. Thus a typical data path for thecommunication involves eNB(s) and/or Gateway(s) (GW(s)) (e.g., SGW/PGW).

FIGS. 7 and 8 are examples of data path scenarios for a proximitycommunication. If wireless devices (e.g., UE1, UE2) are in proximity ofeach other, they may be able to use a direct mode data path (FIG. 7) ora locally routed data path (FIG. 8). In the direct mode data path,wireless devices are connected directly each other (after appropriateprocedure(s), such as authentication), without eNB and SGW/PGW. In thelocally routed data path, wireless devices are connected each otherthrough eNB only.

FIG. 9 is a conceptual diagram illustrating for a non-roaming referencearchitecture.

PC1 to PC 5 represent interfaces. PC1 is a reference point between aProSe application in a UE and a ProSe App server. It is used to defineapplication level signaling requirements. PC 2 is a reference pointbetween the ProSe App Server and the ProSe Function. It is used todefine the interaction between ProSe App Server and ProSe functionalityprovided by the 3GPP EPS via ProSe Function. One example may be forapplication data updates for a ProSe database in the ProSe Function.Another example may be data for use by ProSe App Server in interworkingbetween 3GPP functionality and application data, e.g. name translation.PC3 is a reference point between the UE and ProSe Function. It is usedto define the interaction between UE and ProSe Function. An example maybe to use for configuration for ProSe discovery and communication. PC4is a reference point between the EPC and ProSe Function. It is used todefine the interaction between EPC and ProSe Function. Possible usecases may be when setting up a one-to-one communication path between UEsor when validating ProSe services (authorization) for session managementor mobility management in real time.

PC5 is a reference point between UE to UE used for control and userplane for discovery and communication, for relay and one-to-onecommunication (between UEs directly and between UEs over LTE-Uu).Lastly, PC6 is a reference point may be used for functions such as ProSeDiscovery between users subscribed to different PLMNs.

EPC (Evolved Packet Core) includes entities such as MME, S-GW, P-GW,PCRF, HSS etc. The EPC here represents the E-UTRAN Core Networkarchitecture. Interfaces inside the EPC may also be impacted albeit theyare not explicitly shown in FIG. 9.

Application servers, which are users of the ProSe capability forbuilding the application functionality, e.g. in the Public Safety casesthey can be specific agencies (PSAP) or in the commercial cases socialmedia. These applications are defined outside the 3GPP architecture butthere may be reference points towards 3GPP entities. The Applicationserver can communicate towards an application in the UE.

Applications in the UE use the ProSe capability for building theapplication functionality. Example may be for communication betweenmembers of Public Safety groups or for social media application thatrequests to find buddies in proximity. The ProSe Function in the network(as part of EPS) defined by 3GPP has a reference point towards the ProSeApp Server, towards the EPC and the UE.

The functionality may include but not restricted to e.g.:

-   -   Interworking via a reference point towards the 3rd party        Applications    -   Authorization and configuration of the UE for discovery and        Direct communication    -   Enable the functionality of the EPC level ProSe discovery    -   ProSe related new subscriber data and/handling of data storage;        also handling of ProSe identities;    -   Security related functionality    -   Provide Control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of EPC, e.g.        offline charging)

Especially, the following identities are used for ProSe DirectCommunication:

-   -   Source Layer-2 ID identifies a sender of a D2D packet at PC5        interface. The Source Layer-2 ID is used for identification of        the receiver RLC UM entity;    -   Destination Layer-2 ID identifies a target of the D2D packet at        PC5 interface. The Destination Layer-2 ID is used for filtering        of packets at the MAC layer. The Destination Layer-2 ID may be a        broadcast, groupcast or unicast identifier; and    -   SA L1 ID identifier in Scheduling Assignment (SA) at PC5        interface. SA L1 ID is used for filtering of packets at the        physical layer. The SA L1 ID may be a broadcast, groupcast or        unicast identifier.

No Access Stratum signaling is required for group formation and toconfigure Source Layer-2 ID and Destination Layer-2 ID in the UE. Thisinformation is provided by higher layers.

In case of groupcast and unicast, the MAC layer will convert the higherlayer ProSe ID (i.e. ProSe Layer-2 Group ID and ProSe UE ID) identifyingthe target (Group, UE) into two bit strings of which one can beforwarded to the physical layer and used as SA L1 ID whereas the otheris used as Destination Layer-2 ID. For broadcast, L2 indicates to L1that it is a broadcast transmission using a pre-defined SA L1 ID in thesame format as for group- and unicast.

FIG. 10 is a conceptual diagram illustrating for a Layer 2 structure forSidelink.

The Sidelink is UE to UE interface for ProSe direct communication andProSe Direct Discovery. Corresponds to the PC5 interface. The Sidelinkcomprises ProSe Direct Discovery and ProSe Direct Communication betweenUEs. The Sidelink uses uplink resources and physical channel structuresimilar to uplink transmissions. However, some changes, noted below, aremade to the physical channels. E-UTRA defines two MAC entities; one inthe UE and one in the E-UTRAN. These MAC entities handle the followingtransport channels additionally, i) sidelink broadcast channel (SL-BCH),ii) sidelink discovery channel (SL-DCH) and iii) sidelink shared channel(SL-SCH).

-   -   Basic transmission scheme: the Sidelink transmission uses the        same basic transmission scheme as the UL transmission scheme.        However, sidelink is limited to single cluster transmissions for        all the sidelink physical channels. Further, sidelink uses a 1        symbol gap at the end of each sidelink sub-frame.    -   Physical-layer processing: the Sidelink physical layer        processing of transport channels differs from UL transmission in        the following steps:

i) Scrambling: for PSDCH and PSCCH, the scrambling is not UE-specific;

ii) Modulation: 64 QAM is not supported for Sidelink.

-   -   Physical Sidelink control channel: PSCCH is mapped to the        Sidelink control resources. PSCCH indicates resource and other        transmission parameters used by a UE for PSSCH.    -   Sidelink reference signals: for PSDCH, PSCCH and PSSCH        demodulation, reference signals similar to uplink demodulation        reference signals are transmitted in the 4th symbol of the slot        in normal CP and in the 3rd symbol of the slot in extended        cyclic prefix. The Sidelink demodulation reference signals        sequence length equals the size (number of sub-carriers) of the        assigned resource. For PSDCH and PSCCH, reference signals are        created based on a fixed base sequence, cyclic shift and        orthogonal cover code.    -   Physical channel procedure: for in-coverage operation, the power        spectral density of the sidelink transmissions can be influenced        by the eNB.

FIG. 11a is a conceptual diagram illustrating for User-Plane protocolstack for ProSe Direct Communication, and FIG. 11b is Control-Planeprotocol stack for ProSe Direct Communication.

FIG. 11a shows the protocol stack for the user plane, where PDCP, RLCand MAC sublayers (terminate at the other UE) perform the functionslisted for the user plane (e.g. header compression, HARQretransmissions). The PC5 interface consists of PDCP, RLC, MAC and PHYas shown in FIG. 11 a.

User plane details of ProSe Direct Communication: i) MAC sub headercontains LCIDs (to differentiate multiple logical channels), ii) The MACheader comprises a Source Layer-2 ID and a Destination Layer-2 ID, iii)At MAC Multiplexing/demultiplexing, priority handling and padding areuseful for ProSe Direct communication, iv) RLC UM is used for ProSeDirect communication, v) Segmentation and reassembly of RLC SDUs areperformed, vi) A receiving UE needs to maintain at least one RLC UMentity per transmitting peer UE, vii) An RLC UM receiver entity does notneed to be configured prior to reception of the first RLC UM data unit,and viii) U-Mode is used for header compression in PDCP for ProSe DirectCommunication.

FIG. 11b shows the protocol stack for the control plane, where RRC, RLC,MAC, and PHY sublayers (terminate at the other UE) perform the functionslisted for the control plane. A D2D UE does not establish and maintain alogical connection to receiving D2D UEs prior to a D2D communication.

FIG. 12 is a conceptual diagram illustrating for a PC5 interface forProSe Direct Discovery.

ProSe Direct Discovery is defined as the procedure used by theProSe-enabled UE to discover other ProSe-enabled UE(s) in its proximityusing E-UTRA direct radio signals via PC5.

Radio Protocol Stack (AS) for ProSe Direct Discovery is shown in FIG.12.

The AS layer performs the following functions:

-   -   Interfaces with upper layer (ProSe Protocol): The MAC layer        receives the discovery information from the upper layer (ProSe        Protocol). The IP layer is not used for transmitting the        discovery information.    -   Scheduling: The MAC layer determines the radio resource to be        used for announcing the discovery information received from        upper layer.    -   Discovery PDU generation: The MAC layer builds the MAC PDU        carrying the discovery information and sends the MAC PDU to the        physical layer for transmission in the determined radio        resource. No MAC header is added.

There are two types of resource allocation for discovery informationannouncement.

-   -   Type 1: A resource allocation procedure where resources for        announcing of discovery information are allocated on a non UE        specific basis, further characterized by: i) The eNB provides        the UE(s) with the resource pool configuration used for        announcing of discovery information. The configuration may be        signalled in SIB, ii) The UE autonomously selects radio        resource(s) from the indicated resource pool and announce        discovery information, iii) The UE can announce discovery        information on a randomly selected discovery resource during        each discovery period.    -   Type 2: A resource allocation procedure where resources for        announcing of discovery information are allocated on a per UE        specific basis, further characterized by: i) The UE in        RRC_CONNECTED may request resource(s) for announcing of        discovery information from the eNB via RRC, ii) The eNB assigns        resource(s) via RRC, iii) The resources are allocated within the        resource pool that is configured in UEs for monitoring.

For UEs in RRC_IDLE, the eNB may select one of the following options:

-   -   The eNB may provide a Type 1 resource pool for discovery        information announcement in SIB. UEs that are authorized for        Prose Direct Discovery use these resources for announcing        discovery information in RRC_IDLE.    -   The eNB may indicate in SIB that it supports D2D but does not        provide resources for discovery information announcement. UEs        need to enter RRC Connected in order to request D2D resources        for discovery information announcement.

For UEs in RRC_CONNECTED,

-   -   A UE authorized to perform ProSe Direct Discovery announcement        indicates to the eNB that it wants to perform D2D discovery        announcement.    -   The eNB validates whether the UE is authorized for ProSe Direct        Discovery announcement using the UE context received from MME.    -   The eNB may configure the UE to use a Type 1 resource pool or        dedicated Type 2 resources for discovery information        announcement via dedicated RRC signaling (or no resource).    -   The resources allocated by the eNB are valid until a) the eNB        de-configures the resource(s) by RRC signaling or b) the UE        enters IDLE. (FFS whether resources may remain valid even in        IDLE).

Receiving UEs in RRC_IDLE and RRC_CONNECTED monitor both Type 1 and Type2 discovery resource pools as authorized. The eNB provides the resourcepool configuration used for discovery information monitoring in SIB. TheSIB may contain discovery resources used for announcing in neighborcells as well.

FIG. 13 is a conceptual diagram illustrating for model of twounacknowledged mode peer entities.

In UM (Unacknowledged Mode), in-sequence delivery to higher layers isprovided, but no retransmissions of missing PDUs are requested. UM istypically used for services such as VoIP where error-free delivery is ofless importance compared to short delivery time. TM (Transparent Mode),although supported, is only used for specific purposes such as randomaccess.

Unacknowledged mode (UM) supports segmentation/reassembly andin-sequence delivery, but not retransmissions. This mode is used whenerror-free delivery is not required, for example voice-over IP, or whenretransmissions cannot be requested, for example broadcast transmissionson MTCH and MCCH using MBSFN.

When a transmitting UM RLC entity forms UMD PDUs from RLC SDUs, thetransmitting UM RLC entity may i) segment and/or concatenate the RLCSDUs so that the UMD PDUs fit within the total size of RLC PDU(s)indicated by lower layer at the particular transmission opportunitynotified by lower layer; and ii) include relevant RLC headers in the UMDPDU.

When a receiving UM RLC entity receives UMD PDUs, the receiving UM RLCentity may i) detect whether or not the UMD PDUs have been received induplication, and discard duplicated UMD PDUs; ii) reorder the UMD PDUsif they are received out of sequence; iii) detect the loss of UMD PDUsat lower layers and avoid excessive reordering delays; iv) reassembleRLC SDUs from the reordered UMD PDUs (not accounting for RLC PDUs forwhich losses have been detected) and deliver the RLC SDUs to upper layerin ascending order of the RLC SN; and v) discard received UMD PDUs thatcannot be reassembled into a RLC SDU due to loss at lower layers of anUMD PDU which belonged to the particular RLC SDU.

At the time of RLC re-establishment, the receiving UM RLC entity mayreassemble RLC SDUs from the UMD PDUs that are received out of sequenceand deliver them to upper layer, if possible; ii) discard any remainingUMD PDUs that could not be reassembled into RLC SDUs; and iii)initialize relevant state variables and stop relevant timers.

FIGS. 14a to 14f are conceptual diagrams illustrating for UMD PDU.

FIG. 14a is a diagram for a UMD PDU with 5 bit SN, FIG. 14b is a diagramfor a UMD PDU with 10 bit SN, FIG. 14c is a diagram for a UMD PDU with 5bit SN (Odd number of LIs, i.e. K=1, 3, 5, . . . ), FIG. 14d is adiagram for a UMD PDU with 5 bit SN (Even number of LIs, i.e. K=2, 4, 6,. . . ), FIG. 14e is a UMD PDU with 10 bit SN (Odd number of LIs, i.e.K=1, 3, 5, . . . ), and FIG. 14f is a diagram for a UMD PDU with 10 bitSN (Even number of LIs, i.e. K=2, 4, 6, . . . ).

UMD PDU consists of a Data field and an UMD PDU header. UMD PDU headerconsists of a fixed part (fields that are present for every UMD PDU) andan extension part (fields that are present for an UMD PDU whennecessary). The fixed part of the UMD PDU header itself is byte alignedand consists of a FI, an E and a SN. The extension part of the UMD PDUheader itself is byte aligned and consists of E(s) and LI(s).

An UM RLC entity is configured by RRC to use either a 5 bit SN or a 10bit SN. When the 5 bit SN is configured, the length of the fixed part ofthe UMD PDU header is one byte. When the 10 bit SN is configured, thefixed part of the UMD PDU header is identical to the fixed part of theAMD PDU header, except for D/C, RF and P fields all being replaced withR1 fields. The extension part of the UMD PDU header is identical to theextension part of the AMD PDU header (regardless of the configured SNsize).

An UMD PDU header consists of an extension part only when more than oneData field elements are present in the UMD PDU, in which case an E and aLI are present for every Data field element except the last.Furthermore, when an UMD PDU header consists of an odd number of LI(s),four padding bits follow after the last LI.

In the definition of each field in FIG. 14a to FIG. 14f , the bits inthe parameters are represented in which the first and most significantbit is the left most bit and the last and least significant bit is therightmost bit. Unless mentioned otherwise, integers are encoded instandard binary encoding for unsigned integers.

-   -   Data field: The Data field elements are mapped to the Data field        in the order which they arrive to the RLC entity at the        transmitter. The granularity of the Data field size is one byte;        and the maximum Data field size is the maximum TB size minus the        sum of minimum MAC PDU header size and minimum RLC PDU header        size. A UMD PDU segment is mapped to the Data field. Zero RLC        SDU segments and one or more RLC SDUs, one or two RLC SDU        segments and zero or more RLC SDUs; the RLC SDU segments are        either mapped to the beginning or the end of the Data field, a        RLC SDU or RLC SDU segment larger than 2047 octets can only be        mapped to the end of the Data field. When there are two RLC SDU        segments, they belong to different RLC SDUs.    -   Sequence number (SN) field: the SN field indicates the sequence        number of the corresponding UMD or AMD PDU. For an AMD PDU        segment, the SN field indicates the sequence number of the        original AMD PDU from which the AMD PDU segment was constructed        from. The sequence number is incremented by one for every UMD or        AMD PDU. Length is 5 bits or 10 bits (configurable) for UMD PDU.    -   Extension bit (E) field: Length is 1 bit. The E field indicates        whether Data field follows or a set of E field and LI field        follows. The interpretation of the E field is provided in Table        1 and Table 2.

TABLE 1 Value Description 0 Data field follows from the octet followingthe fixed part of the header 1 A set of E field and LI field followsfrom the octet following the fixed part of the header

TABLE 2 Value Description 0 Data field follows from the octet followingthe LI field following this E field 1 A set of E field and LI fieldfollows from the bit following the LI field following this E field

-   -   Length Indicator (LI) field: Length is 11 bits. The LI field        indicates the length in bytes of the corresponding Data field        element present in the RLC data PDU delivered/received by an UM        or an AM RLC entity. The first LI present in the RLC data PDU        header corresponds to the first Data field element present in        the Data field of the RLC data PDU, the second LI present in the        RLC data PDU header corresponds to the second Data field element        present in the Data field of the RLC data PDU, and so on. The        value 0 is reserved.    -   Framing Info (FI) field: Length is 2 bits. The FI field        indicates whether a RLC SDU is segmented at the beginning and/or        at the end of the Data field. Specifically, the FI field        indicates whether the first byte of the Data field corresponds        to the first byte of a RLC SDU, and whether the last byte of the        Data field corresponds to the last byte of a RLC SDU. The        interpretation of the FI field is provided in Table 3.

TABLE 3 Value Description 00 First byte of the Data field corresponds tothe first byte of a RLC SDU. Last byte of the Data field corresponds tothe last byte of a RLC SDU. 01 First byte of the Data field correspondsto the first byte of a RLC SDU. Last byte of the Data field does notcorrespond to the last byte of a RLC SDU. 10 First byte of the Datafield does not correspond to the first byte of a RLC SDU. Last byte ofthe Data field corresponds to the last byte of a RLC SDU. 11 First byteof the Data field does not correspond to the first byte of a RLC SDU.Last byte of the Data field does not correspond to the last byte of aRLC SDU.

FIG. 15 is a conceptual diagram illustrating for UM RLC reorderingwindow operation.

Each transmitting UM RLC entity shall maintain the following statevariables in this application:

a) VT(US): this state variable holds the value of the SN to be assignedfor the next newly generated UMD PDU. It is initially set to 0, and isupdated whenever the UM RLC entity delivers an UMD PDU with SN=VT(US).

Each receiving UM RLC entity shall maintain the following statevariables in this application:

a) VR(UR)—UM receive state variable: this state variable holds the valueof the SN of the earliest UMD PDU that is still considered forreordering. It is initially set to 0.

b) VR(UX)—UM t-Reordering state variable: this state variable holds thevalue of the SN following the SN of the UMD PDU which triggeredt-Reordering.

c) VR(UH)—UM highest received state variable: this state variable holdsthe value of the SN following the SN of the UMD PDU with the highest SNamong received UMD PDUs, and it serves as the higher edge of thereordering window. It is initially set to 0.

The receiving UM RLC entity may maintain a reordering window accordingto state variable VR(UH) as follows:

i) a SN falls within the reordering window if(VR(UH)−UM_Window_Size)≤SN<VR(UH);

ii) a SN falls outside of the reordering window otherwise.

When receiving an UMD PDU from lower layer, the receiving UM RLC entitymay either discard the received UMD PDU or place it in the receptionbuffer.

If the received UMD PDU was placed in the reception buffer, thereceiving UM RLC may update state variables, reassemble and deliver RLCSDUs to upper layer and start/stop t-Reordering as needed.

When t-Reordering expires, the receiving UM RLC entity may update statevariables, reassemble and deliver RLC SDUs to upper layer and startt-Reordering as needed

When an UMD PDU with SN=x is received from lower layer, the receiving UMRLC entity may discard the received UMD PDU, if VR(UR)<x<VR(UH) and theUMD PDU with SN=x has been received before; or if(VR(UH)−UM_Window_Size)≤x<VR(UR).

Else, the receiving UM RLC entity may place the received UMD PDU in thereception buffer.

When an UMD PDU with SN=x is placed in the reception buffer, thereceiving UM RLC entity may update VR(UH) to x+1 and reassemble RLC SDUsfrom any UMD PDUs with SN that falls outside of the reordering window,remove RLC headers when doing so and deliver the reassembled RLC SDUs toupper layer in ascending order of the RLC SN if not delivered before, ifx falls outside of the reordering window.

If VR(UR) falls outside of the reordering window, the receiving UM RLCentity may set VR(UR) to (VR(UH)−UM_Window_Size).

If the reception buffer contains an UMD PDU with SN=VR(UR), thereceiving UM RLC entity may update VR(UR) to the SN of the first UMD PDUwith SN>current VR(UR) that has not been received; and reassemble RLCSDUs from any UMD PDUs with SN<updated VR(UR), remove RLC headers whendoing so and deliver the reassembled RLC SDUs to upper layer inascending order of the RLC SN if not delivered before;

If t-Reordering is running and VR(UX)≤VR(UR); or if t-Reordering isrunning and VR(UX) falls outside of the reordering window and VR(UX) isnot equal to VR(UH), the receiving UM RLC entity may stop and resett-Reordering.

If t-Reordering is not running (includes the case when t-Reordering isstopped due to actions above) and VR(UH)>VR(UR), the receiving UM RLCentity may start t-Reordering, and set VR(UX) to VR(UH).

When t-Reordering expires, the receiving UM RLC entity may update VR(UR)to the SN of the first UMD PDU with SN≥VR(UX) that has not beenreceived; and reassemble RLC SDUs from any UMD PDUs with SN<updatedVR(UR), remove RLC headers when doing so and deliver the reassembled RLCSDUs to upper layer in ascending order of the RLC SN if not deliveredbefore.

If VR(UH)>VR(UR), the receiving UM RLC entity may start t-Reordering,and set VR(UX) to VR(UH).

For example, the received PDU is either reordered or discarded dependingon the comparison result of received RLC SN and state variables, asexplained in FIG. 14.

If a RLC PDU with SN=X is received, the UM RLC compares the X with statevariables.

-   -   If the X is within region 1, i.e. VR(UH)−UM_WIN<=X<VR(UR), the        UE considers it as old packet, and discards it.    -   If the X is within region 2, i.e. VR(UR)<=X<VR(UH), the UE        reorders it if it is not duplicate. Otherwise, the UE discards        it.    -   If the X is within region 3, i.e. VR(UH)<=X, the UE considers it        as new packet, and stores it in the reordering buffer. The UE        advances reordering window by setting VR(UH) to X+1.

FIG. 16 is a conceptual diagram for processing RLC PDU for D2Dcommunication according to embodiments of the present invention.

In order to synchronize the RLC and PDCP state variables between thetransmitter and the receiver at the RLC and PDCP entity establishment inthe receiver, the following methods are invented.

In RLC entity, when a receiver receives a first RLC PDU for a UM RLCentity from a transmitter, it establishes the UM RLC entity and sets theUM RLC state variables, VR(UH) and VR(UR), to the RLC sequence number(SN) of the first received RLC PDU for the UM RLC entity.

When a receiving UE (rx UE) receives a first RLC PDU for a RLC entityfrom a transmitting UE (tx UE) (S1601), the receiving UE establishes theRLC entity to process the first RLC PDU (S1603).

Preferably, the RLC entity established belongs to a sidelink radiobearer above mentioned.

Preferably, the first RLC PDU is an RLC PDU received firstly before anyother RLC PDUs are received by the RLC entity from the transmitting UE.

After the step of S1603, the receiving UE sets state variables (e.g,VR(UR) and VR(UH)) for the RLC entity to a RLC sequence number (SN) ofthe first RLC PDU for the RLC entity (S1605).

Preferably, the VR(UR) is a received state variable for a UM RLC entityand holds a value of RLC SN of a earliest UMD PDU that is stillconsidered for reordering. For RLC entity configured for STCH, it isinitially set to the SN of the first received UMD PDU.

Preferably, the VR(UH) is highest received state variable for the UM RLCentity and holds a value of RLC SN following SN of a UMD PDU with thehighest SN among received UMD PDUs. For RLC entity configured for STCH,it is initially set to the SN of the first received UMD PDU.

In other words, the UM RLC state variables, VR(UH) and VR(UR), areinitialized to the RLC SN of the first received UMD PDU for the UM RLCentity.

After initializing the VR(UR) and VR(UH) to the RLC SN of the firstreceived UMD PDU, the receiver processes the UMD PDU using theinitialized VR(UR) and VR(UH) (S1607). Since the first received SN isequal to VR(UR) and VR(UH), the first received UMD PDU is considered tobe within the receiving window, and thus it is not discarded accordingto the following procedure.

When an UMD PDU with SN=x is received from lower layer, the receivingRLC entity may discard the received UMD PDU, if VR(UR)<x<VR(UH) and theUMD PDU with SN=x has been received before; or if(VR(UH)−UM_Window_Size) ≤x<VR(UR). Otherwise, the receiving RLC entitymay place the received UMD PDU in the reception buffer.

Preferably, the RLC entity may be a UM (Unacknowledged Mode)-RLC entity.

Preferably, the RLC PDU is an RLC UMD (Un Unacknowledged Mode Data) PDU.

In PDCP, when a receiver receives a first PDCP PDU from a transmitter,it establishes the PDCP entity and sets the PDCP state variable,Next_PDCP_RX_SN, to the PDCP sequence number (SN) of the first receivedPDCP PDU for the PDCP entity. In other words, the PDCP state variable,Next_PDCP_RX_SN, is initialized to the PDCP SN of the first receivedPDCP PDU for the PDCP entity.

The variable Next_PDCP_RX_SN indicates the next expected PDCP SN by thereceiver for a given PDCP entity.

However, the receiver does not know the correct RX_HFN value, and thedeciphering would not succeed unless the receiver sets the RX_HFN to thecorrect value. For the receiver to synchronize the HFN value to thetransmitter, it is therefore invented that a fixed value of HFN is usedfor the D2D communication. The fixed value of HFN can be signaled to theUE by the eNB when the eNB configures D2D configuration to the UE, orcan be fixed in the specification. The transmitter and the receiver PDCPentity set the HFN value to the fixed value at establishment, and usethe fixed HFN value for ciphering/deciphering of all the PDCP PDUs.

For the transmitter to avoid HFN increment due to PDCP SN wrap around,it is also invented that the transmitter releases the used PDCP entityand establishes a new PDCP entity if the PDCP SN reaches the maximumvalue of the PDCP SN.

After initializing the Next_PDCP_RX_SN to the PDCP SN of the firstreceived PDCP PDU and RX_HFN to the fixed value, the receiver processesthe received PDCP PDU using the initialized Next_PDCP_RX_SN and RX_HFN.

FIG. 17 is a conceptual diagram for processing RLC PDU for D2Dcommunication according to embodiments of the present invention.

Regarding FIG. 15 above mentioned, the initial values of the statevariables VR(UH) and VR(UR) are zero, and the size of UM windowUM_WIN=16 for 5 bit SN and 512 for 10 bit SN. Then, when a UM RLC entityis established, the UM RLC entity will discard the received packet if ithas the RLC SN between 16 to 31 (for 5 bit SN) or 512 to 1023 (for 10bit SN). The UM RLC entity will keep discarding the packet until thereceived packet has the RLC SN between 0 to 15 (for 5 bit SN) or 0 to511 (for 10 bit SN).

Considering that the UE can join the D2D group at any time duringcommunication, the first received PDU can have any value of SN, and itis required to avoid packet discarding at RLC entity establishment.

To avoid packet discarding at RLC entity establishment, the UE may set aplurality of state variables for the RLC entity to initial values when afirst RLC PDU is received from a peer UE.

When a receiving UE (rx UE) receives a first RLC PDU for a RLC entityfrom a transmitting UE (tx UE) (S1701), the receiving UE establishes theRLC entity to process the first RLC PDU (S1703).

Preferably, the RLC entity established belongs to a sidelink radiobearer above mentioned.

Preferably, the first RLC PDU is an RLC PDU received firstly before anyother RLC PDUs are received by the RLC entity from the transmitting UE.

Preferably, the RLC entity is a UM (Unacknowledged Mode)-RLC entity andthe RLC PDU is an RLC UMD (Un Unacknowledged Mode Data) PDU.

After the step of S1703, the receiving UE sets a plurality of statevariables for the RLC entity to initial values (S1705).

The plurality of state variables comprises VR(UH) and VR(UR). the VR(UR)is a received state variable for a UM RLC entity and holds a value ofRLC SN of a earliest UMD PDU that is still considered for reordering,and the VR(UH) is highest received state variable for the UM RLC entityand holds a value of RLC SN following SN of a UMD PDU with the highestSN among received UMD PDUs.

Preferably, the an initial value of VR(UH) is a RLC sequence number (SN)of the first RLC PDU and an initial value of VR(UR) is ‘the RLC SN ofthe first RLC PDU-offset’.

The receiving UE may process the first RLC PDU using the initial values(S1707).

If the receiving UE receives the first RLC PDU with RLC SN being X, thereceiving UE sets VR(UH) to X, and VR(UR) to ‘X-offset’.

Preferably, the offset value is signaled from the eNB bySIB/RRC/PDCP/RLC/MAC/PHY signaling, or specified in the specification.

When the receiving UE sets the plurality of state variables for the RLCentity to the initial values (S1705), the receiving UE stores the firstreceived PDU in the reordering buffer because VR(UR)<X (S1709).

And then because X falls outside of the reordering window (e.g.,X=VR(UH)), the UE can update VR(UH) to X+1. And set VR(UX) to X+1because, VR(UH)>VR(UR) (S1711).

At the step of S1709, t-reordering timer can start. And if the timerexpires, the receiving UE delivers the first RLC PDU to a PDCP entity(S1713).

The steps of S1701 to S1713 are a slight enhancement of the steps ofS1601 to S1607. In the steps of S1601 to S1607, if the second andfollowing PDUs have SNs lower than the SN of the first PDU, they are alldiscarded as the SNs of them are lower than VR(UR). The steps of S1701to S1713 are useful when the first PDU is received out-of-order. Ifthere is an offset between the VR(UH) and VR(UR), RLC PDUs received inout-of sequence satisfied a specific condition can be store inreordering buffer.

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

In the embodiments of the present invention, a specific operationdescribed as performed by the BS may be performed by an upper node ofthe BS. Namely, it is apparent that, in a network comprised of aplurality of network nodes including a BS, various operations performedfor communication with an MS may be performed by the BS, or networknodes other than the BS. The term ‘eNB’ may be replaced with the term‘fixed station’, ‘Node B’, ‘Base Station (BS)’, ‘access point’, etc.

The above-described embodiments may be implemented by various means, forexample, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments ofthe present invention may be implemented by one or more ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to theembodiments of the present invention may be implemented in the form ofmodules, procedures, functions, etc. performing the above-describedfunctions or operations. Software code may be stored in a memory unitand executed by a processor. The memory unit may be located at theinterior or exterior of the processor and may transmit and receive datato and from the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the above-described method has been described centering on anexample applied to the 3GPP LTE system, the present invention isapplicable to a variety of wireless communication systems in addition tothe 3GPP LTE system.

The invention claimed is:
 1. A method for a User Equipment (UE)operating in a wireless communication system, the method comprising:receiving a first Radio Link Control (RLC) Protocol Data Unit (PDU) froma peer UE via a direct interface associated with a sidelink trafficchannel, the RLC PDU including a Packet Data Convergence Protocol (PDCP)Service Data Unit (SDU); establishing a RLC entity and a PDCP entityconfigured for the sidelink traffic channel, when the first RLC PDU isreceived from the peer UE, wherein, when the RLC entity is newlyestablished, an initial value of VR(UH) for the RLC entity is set to aRLC sequence number (SN) of the first RLC PDU, and an initial value ofVR(UR) for the RLC entity is set to ‘the RLC SN of the first RLCPDU-offset’; processing, at the RLC entity, the first RLC PDU using theinitial values; and deciphering, at the PDCP entity, the PDCP SDU basedon a Hyper Frame Number (HFN) and a PDCP SN of the PDCP SDU, wherein theHFN has a fixed value for the sidelink traffic channel not incrementedwhen the PDCP SN reaches to a maximum value.
 2. The method according toclaim 1, wherein the RLC entity is a Unacknowledged Mode (UM)-RLCentity.
 3. The method according to claim 1, wherein the first RLC PDU isan RLC Unacknowledged Mode Data (UMD) PDU.
 4. The method according toclaim 1, wherein the VR(UR) is a received state variable for a UM RLCentity and holds a value of RLC SN of a earliest UMD PDU that is stillconsidered for reordering, and wherein the VR(UH) is highest receivedstate variable for the UM RLC entity and holds a value of RLC SNfollowing SN of a UMD PDU with the highest SN among received UMD PDUs.5. The method according to claim 1, wherein the first RLC PDU is an RLCPDU received firstly before any other RLC PDUs are received by the RLCentity from the peer UE.
 6. The method according to claim 1, furthercomprising: setting the VR(UH) to ‘the RLC SN of the first RLC PDU +1’,when the first RLC PDU is stored in a reordering buffer.
 7. The methodaccording to claim 1, further comprising: starting a timer when thefirst RLC PDU is stored in a reordering buffer; and setting a UR(UX) to‘the RLC SN of the first RLC PDU +1’, wherein the UR(UX) holds a valueof SN following SN of a RLC PDU which triggered the timer.
 8. The methodaccording to claim 7, further comprising: delivering the first RLC PDUto a Packet Data Convergence Protocol (PDCP) entity when the timerexpires.
 9. The method according to claim 1, further comprising:receiving a value of the offset from the peer UE or a eNode-B by SystemInformation Blocks (SIBs), Radio Resource Control (RRC), Packet DataConvergence Protocol (PDCP), Medium Access Control (MAC) or PHYsical(PHY) signaling.
 10. A User Equipment (UE) operating in a wirelesscommunication system, the UE comprising: a Radio Frequency (RF) module;and a processor configured to control the RF module, receive a firstRadio Link Control (RLC) Protocol Data Unit (PDU) from a peer UE via adirect interface associated with a sidelink traffic channel, the RLC PDUincluding a Packet Data Convergence Protocol (PDCP) Service Data Unit(SDU); establish a RLC entity and a PDCP entity configured for thesidelink traffic channel, when the first RLC PDU is received from thepeer UE, wherein when the RLC entity is newly established, an initialvalue of VR(UH) for the RLC entity is set to a RLC sequence number (SN)of the first RLC PDU, and an initial value of VR(UR) for the RLC entityis set to ‘the RLC SN of the first RLC PDU-offset’, process the firstRLC PDU using the initial values; and decipher, at the PDCP entity, thePDCP SDU based on a Hyper Frame Number (HFN) and a PDCP SN of the PDCPSDU, wherein the HFN has a fixed value for the sidelink traffic channelnot incremented when the PDCP SN reaches to a maximum value.
 11. The UEaccording to claim 10, wherein the RLC entity is a UM UnacknowledgedMode (UM)-RLC entity.
 12. The UE according to claim 10, wherein the RLCPDU is an RLC Un Unacknowledged Mode Data (UMD) PDU.
 13. The UEaccording to claim 10, wherein the VR(UR) is a received state variablefor a UM RLC entity and holds a value of RLC SN of a earliest UMD PDUthat is still considered for reordering, and wherein the VR(UH) ishighest received state variable for the UM RLC entity and holds a valueof RLC SN following SN of a UMD PDU with the highest SN among receivedUMD PDUs.
 14. The UE according to claim 10, wherein the first RLC PDU isan RLC PDU received firstly before any other RLC PDUs are received bythe RLC entity from the peer UE.
 15. The UE according to claim 10,wherein the UE sets the VR(UH) to ‘the RLC SN of the first RLC PDU +1’,when the first RLC PDU is stored in a reordering buffer.
 16. The UEaccording to claim 10, wherein the processor is further configured tostart a timer when the first RLC PDU is stored in a reordering buffer,and to set a UR(UX) to ‘the RLC SN of the first RLC PDU +1’, wherein theUR(UX) holds a value of SN following SN of a RLC PDU which triggered thetimer.
 17. The UE according to claim 16, wherein the processor isfurther configured to deliver the first RLC PDU to a Packet DataConvergence Protocol (PDCP) entity when the timer expires.
 18. The UEaccording to claim 10, wherein the processor is further configured toreceive a value the offset from the peer UE or a eNode-B by SystemInformation Blocks (SIBs), Radio Resource Control (RRC), Packet DataConvergence Protocol (PDCP), Medium Access Control (MAC) or physicalsignaling.